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          每天进步一点点016 - sync.Map
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            <div class="post-description">了解一下 sync.Map 的实现</div>

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        <h1 id="Preface"><a href="#Preface" class="headerlink" title="Preface"></a>Preface</h1><p>之前面试有遇到过普通 <strong><em>Map</em></strong> 的安全问题，其实也就是问关于数据竞争的问题。<br><em>golang</em> 后面推出的 <em>sync.Map</em> 就是用于解决这个问题的。<br>当时对方提到也可以通过 <em>chan</em> 来做 <em>map</em> 的数据读写，这样就可以保证数据的正确性。<br>个人一直对这个做法表示怀疑态度，<em>channel</em> 用于此使用率很高的对象，会不会造成滥用或者性能方面的问题呢？暂时没时间去了解。<br>当初个人给出 <em>map</em> 配锁的方案，面试官问到需要如何保证使用者在用完之后释放锁，从而避免死锁。<br>个人当初也提出了自己的解决方案：自制一个<em>map*，配合以 *mutex</em> 来保证其安全，将加解锁的操作全部限制在这个自定义的 <em>map</em> 内。<br>普通开发人员只需要直接的使用就可以了。<br>不曾想，大概的看来一下 <em>sync.Map</em> 之后其中也是依赖 <em>mutex</em> 来确保数据的安全的。<br>当然，个人认为，这种普通对象的使用，更多的要从管理上去解决，诸如：面试的时候多注意员工的编程基础，公司提供完善的编码规范，完善的api，以及多对员工进行一些培训之类的。<br>千人千面，员工难免良莠不齐，但是很多事情可以通过一些手段去解决的。<br>不纠结，开始看代码。</p>
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class="line">359</span><br><span class="line">360</span><br><span class="line">361</span><br><span class="line">362</span><br><span class="line">363</span><br><span class="line">364</span><br><span class="line">365</span><br><span class="line">366</span><br><span class="line">367</span><br><span class="line">368</span><br><span class="line">369</span><br><span class="line">370</span><br><span class="line">371</span><br><span class="line">372</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// Copyright 2016 The Go Authors. All rights reserved.</span></span><br><span class="line"><span class="comment">// Use of this source code is governed by a BSD-style</span></span><br><span class="line"><span class="comment">// license that can be found in the LICENSE file.</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">package</span> sync</span><br><span class="line"></span><br><span class="line"><span class="keyword">import</span> (</span><br><span class="line">	<span class="string">&quot;sync/atomic&quot;</span></span><br><span class="line">	<span class="string">&quot;unsafe&quot;</span></span><br><span class="line">)</span><br><span class="line"></span><br><span class="line"><span class="comment">// Map 与 Go 的 map[interface&#123;&#125;]interface&#123;&#125; 很像，不同的是他是并发安全的，在多协程并发中使用他可以不用额外加锁或者其他的调整</span></span><br><span class="line"><span class="comment">// 载入, 存放, 和 删除操作消耗同样的时间</span></span><br><span class="line"><span class="comment">//</span></span><br><span class="line"><span class="comment">// The Map type is specialized. Most code should use a plain Go map instead,</span></span><br><span class="line"><span class="comment">// with separate locking or coordination, for better type safety and to make it</span></span><br><span class="line"><span class="comment">// easier to maintain other invariants along with the map content.</span></span><br><span class="line"><span class="comment">//</span></span><br><span class="line"><span class="comment">// The Map type is optimized for two common use cases: (1) when the entry for a given</span></span><br><span class="line"><span class="comment">// key is only ever written once but read many times, as in caches that only grow,</span></span><br><span class="line"><span class="comment">// or (2) when multiple goroutines read, write, and overwrite entries for disjoint</span></span><br><span class="line"><span class="comment">// sets of keys. In these two cases, use of a Map may significantly reduce lock</span></span><br><span class="line"><span class="comment">// contention compared to a Go map paired with a separate Mutex or RWMutex.</span></span><br><span class="line"><span class="comment">//</span></span><br><span class="line"><span class="comment">// The zero Map is empty and ready for use. A Map must not be copied after first use.</span></span><br><span class="line"><span class="keyword">type</span> Map <span class="keyword">struct</span> &#123;</span><br><span class="line">	mu Mutex</span><br><span class="line"></span><br><span class="line">	<span class="comment">// read contains the portion of the map&#x27;s contents that are safe for</span></span><br><span class="line">	<span class="comment">// concurrent access (with or without mu held).</span></span><br><span class="line">	<span class="comment">//</span></span><br><span class="line">	<span class="comment">// The read field itself is always safe to load, but must only be stored with</span></span><br><span class="line">	<span class="comment">// mu held.</span></span><br><span class="line">	<span class="comment">//</span></span><br><span class="line">	<span class="comment">// Entries stored in read may be updated concurrently without mu, but updating</span></span><br><span class="line">	<span class="comment">// a previously-expunged entry requires that the entry be copied to the dirty</span></span><br><span class="line">	<span class="comment">// map and unexpunged with mu held.</span></span><br><span class="line">	read atomic.Value <span class="comment">// readOnly</span></span><br><span class="line"></span><br><span class="line">	<span class="comment">// dirty contains the portion of the map&#x27;s contents that require mu to be</span></span><br><span class="line">	<span class="comment">// held. To ensure that the dirty map can be promoted to the read map quickly,</span></span><br><span class="line">	<span class="comment">// it also includes all of the non-expunged entries in the read map.</span></span><br><span class="line">	<span class="comment">//</span></span><br><span class="line">	<span class="comment">// Expunged entries are not stored in the dirty map. An expunged entry in the</span></span><br><span class="line">	<span class="comment">// clean map must be unexpunged and added to the dirty map before a new value</span></span><br><span class="line">	<span class="comment">// can be stored to it.</span></span><br><span class="line">	<span class="comment">//</span></span><br><span class="line">	<span class="comment">// If the dirty map is nil, the next write to the map will initialize it by</span></span><br><span class="line">	<span class="comment">// making a shallow copy of the clean map, omitting stale entries.</span></span><br><span class="line">	dirty <span class="keyword">map</span>[<span class="keyword">interface</span>&#123;&#125;]*entry</span><br><span class="line"></span><br><span class="line">	<span class="comment">// misses counts the number of loads since the read map was last updated that</span></span><br><span class="line">	<span class="comment">// needed to lock mu to determine whether the key was present.</span></span><br><span class="line">	<span class="comment">//</span></span><br><span class="line">	<span class="comment">// Once enough misses have occurred to cover the cost of copying the dirty</span></span><br><span class="line">	<span class="comment">// map, the dirty map will be promoted to the read map (in the unamended</span></span><br><span class="line">	<span class="comment">// state) and the next store to the map will make a new dirty copy.</span></span><br><span class="line">	misses <span class="keyword">int</span></span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// readOnly is an immutable struct stored atomically in the Map.read field.</span></span><br><span class="line"><span class="keyword">type</span> readOnly <span class="keyword">struct</span> &#123;</span><br><span class="line">	m       <span class="keyword">map</span>[<span class="keyword">interface</span>&#123;&#125;]*entry</span><br><span class="line">	amended <span class="keyword">bool</span> <span class="comment">// true if the dirty map contains some key not in m.</span></span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// expunged is an arbitrary pointer that marks entries which have been deleted</span></span><br><span class="line"><span class="comment">// from the dirty map.</span></span><br><span class="line"><span class="keyword">var</span> expunged = unsafe.Pointer(<span class="built_in">new</span>(<span class="keyword">interface</span>&#123;&#125;))</span><br><span class="line"></span><br><span class="line"><span class="comment">// An entry is a slot in the map corresponding to a particular key.</span></span><br><span class="line"><span class="keyword">type</span> entry <span class="keyword">struct</span> &#123;</span><br><span class="line">	<span class="comment">// p points to the interface&#123;&#125; value stored for the entry.</span></span><br><span class="line">	<span class="comment">//</span></span><br><span class="line">	<span class="comment">// If p == nil, the entry has been deleted and m.dirty == nil.</span></span><br><span class="line">	<span class="comment">//</span></span><br><span class="line">	<span class="comment">// If p == expunged, the entry has been deleted, m.dirty != nil, and the entry</span></span><br><span class="line">	<span class="comment">// is missing from m.dirty.</span></span><br><span class="line">	<span class="comment">//</span></span><br><span class="line">	<span class="comment">// Otherwise, the entry is valid and recorded in m.read.m[key] and, if m.dirty</span></span><br><span class="line">	<span class="comment">// != nil, in m.dirty[key].</span></span><br><span class="line">	<span class="comment">//</span></span><br><span class="line">	<span class="comment">// An entry can be deleted by atomic replacement with nil: when m.dirty is</span></span><br><span class="line">	<span class="comment">// next created, it will atomically replace nil with expunged and leave</span></span><br><span class="line">	<span class="comment">// m.dirty[key] unset.</span></span><br><span class="line">	<span class="comment">//</span></span><br><span class="line">	<span class="comment">// An entry&#x27;s associated value can be updated by atomic replacement, provided</span></span><br><span class="line">	<span class="comment">// p != expunged. If p == expunged, an entry&#x27;s associated value can be updated</span></span><br><span class="line">	<span class="comment">// only after first setting m.dirty[key] = e so that lookups using the dirty</span></span><br><span class="line">	<span class="comment">// map find the entry.</span></span><br><span class="line">	p unsafe.Pointer <span class="comment">// *interface&#123;&#125;</span></span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="title">newEntry</span><span class="params">(i <span class="keyword">interface</span>&#123;&#125;)</span> *<span class="title">entry</span></span> &#123;</span><br><span class="line">	<span class="keyword">return</span> &amp;entry&#123;p: unsafe.Pointer(&amp;i)&#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// Load returns the value stored in the map for a key, or nil if no</span></span><br><span class="line"><span class="comment">// value is present.</span></span><br><span class="line"><span class="comment">// The ok result indicates whether value was found in the map.</span></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="params">(m *Map)</span> <span class="title">Load</span><span class="params">(key <span class="keyword">interface</span>&#123;&#125;)</span> <span class="params">(value <span class="keyword">interface</span>&#123;&#125;, ok <span class="keyword">bool</span>)</span></span> &#123;</span><br><span class="line">	read, _ := m.read.Load().(readOnly)</span><br><span class="line">	e, ok := read.m[key]</span><br><span class="line">	<span class="keyword">if</span> !ok &amp;&amp; read.amended &#123;</span><br><span class="line">		m.mu.Lock()</span><br><span class="line">		<span class="comment">// Avoid reporting a spurious miss if m.dirty got promoted while we were</span></span><br><span class="line">		<span class="comment">// blocked on m.mu. (If further loads of the same key will not miss, it&#x27;s</span></span><br><span class="line">		<span class="comment">// not worth copying the dirty map for this key.)</span></span><br><span class="line">		read, _ = m.read.Load().(readOnly)</span><br><span class="line">		e, ok = read.m[key]</span><br><span class="line">		<span class="keyword">if</span> !ok &amp;&amp; read.amended &#123;</span><br><span class="line">			e, ok = m.dirty[key]</span><br><span class="line">			<span class="comment">// Regardless of whether the entry was present, record a miss: this key</span></span><br><span class="line">			<span class="comment">// will take the slow path until the dirty map is promoted to the read</span></span><br><span class="line">			<span class="comment">// map.</span></span><br><span class="line">			m.missLocked()</span><br><span class="line">		&#125;</span><br><span class="line">		m.mu.Unlock()</span><br><span class="line">	&#125;</span><br><span class="line">	<span class="keyword">if</span> !ok &#123;</span><br><span class="line">		<span class="keyword">return</span> <span class="literal">nil</span>, <span class="literal">false</span></span><br><span class="line">	&#125;</span><br><span class="line">	<span class="keyword">return</span> e.load()</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="params">(e *entry)</span> <span class="title">load</span><span class="params">()</span> <span class="params">(value <span class="keyword">interface</span>&#123;&#125;, ok <span class="keyword">bool</span>)</span></span> &#123;</span><br><span class="line">	p := atomic.LoadPointer(&amp;e.p)</span><br><span class="line">	<span class="keyword">if</span> p == <span class="literal">nil</span> || p == expunged &#123;</span><br><span class="line">		<span class="keyword">return</span> <span class="literal">nil</span>, <span class="literal">false</span></span><br><span class="line">	&#125;</span><br><span class="line">	<span class="keyword">return</span> *(*<span class="keyword">interface</span>&#123;&#125;)(p), <span class="literal">true</span></span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// Store sets the value for a key.</span></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="params">(m *Map)</span> <span class="title">Store</span><span class="params">(key, value <span class="keyword">interface</span>&#123;&#125;)</span></span> &#123;</span><br><span class="line">	read, _ := m.read.Load().(readOnly)</span><br><span class="line">	<span class="keyword">if</span> e, ok := read.m[key]; ok &amp;&amp; e.tryStore(&amp;value) &#123;</span><br><span class="line">		<span class="keyword">return</span></span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">	m.mu.Lock()</span><br><span class="line">	read, _ = m.read.Load().(readOnly)</span><br><span class="line">	<span class="keyword">if</span> e, ok := read.m[key]; ok &#123;</span><br><span class="line">		<span class="keyword">if</span> e.unexpungeLocked() &#123;</span><br><span class="line">			<span class="comment">// The entry was previously expunged, which implies that there is a</span></span><br><span class="line">			<span class="comment">// non-nil dirty map and this entry is not in it.</span></span><br><span class="line">			m.dirty[key] = e</span><br><span class="line">		&#125;</span><br><span class="line">		e.storeLocked(&amp;value)</span><br><span class="line">	&#125; <span class="keyword">else</span> <span class="keyword">if</span> e, ok := m.dirty[key]; ok &#123;</span><br><span class="line">		e.storeLocked(&amp;value)</span><br><span class="line">	&#125; <span class="keyword">else</span> &#123;</span><br><span class="line">		<span class="keyword">if</span> !read.amended &#123;</span><br><span class="line">			<span class="comment">// We&#x27;re adding the first new key to the dirty map.</span></span><br><span class="line">			<span class="comment">// Make sure it is allocated and mark the read-only map as incomplete.</span></span><br><span class="line">			m.dirtyLocked()</span><br><span class="line">			m.read.Store(readOnly&#123;m: read.m, amended: <span class="literal">true</span>&#125;)</span><br><span class="line">		&#125;</span><br><span class="line">		m.dirty[key] = newEntry(value)</span><br><span class="line">	&#125;</span><br><span class="line">	m.mu.Unlock()</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// tryStore stores a value if the entry has not been expunged.</span></span><br><span class="line"><span class="comment">//</span></span><br><span class="line"><span class="comment">// If the entry is expunged, tryStore returns false and leaves the entry</span></span><br><span class="line"><span class="comment">// unchanged.</span></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="params">(e *entry)</span> <span class="title">tryStore</span><span class="params">(i *<span class="keyword">interface</span>&#123;&#125;)</span> <span class="title">bool</span></span> &#123;</span><br><span class="line">	<span class="keyword">for</span> &#123;</span><br><span class="line">		p := atomic.LoadPointer(&amp;e.p)</span><br><span class="line">		<span class="keyword">if</span> p == expunged &#123;</span><br><span class="line">			<span class="keyword">return</span> <span class="literal">false</span></span><br><span class="line">		&#125;</span><br><span class="line">		<span class="keyword">if</span> atomic.CompareAndSwapPointer(&amp;e.p, p, unsafe.Pointer(i)) &#123;</span><br><span class="line">			<span class="keyword">return</span> <span class="literal">true</span></span><br><span class="line">		&#125;</span><br><span class="line">	&#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// unexpungeLocked ensures that the entry is not marked as expunged.</span></span><br><span class="line"><span class="comment">//</span></span><br><span class="line"><span class="comment">// If the entry was previously expunged, it must be added to the dirty map</span></span><br><span class="line"><span class="comment">// before m.mu is unlocked.</span></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="params">(e *entry)</span> <span class="title">unexpungeLocked</span><span class="params">()</span> <span class="params">(wasExpunged <span class="keyword">bool</span>)</span></span> &#123;</span><br><span class="line">	<span class="keyword">return</span> atomic.CompareAndSwapPointer(&amp;e.p, expunged, <span class="literal">nil</span>)</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// storeLocked unconditionally stores a value to the entry.</span></span><br><span class="line"><span class="comment">//</span></span><br><span class="line"><span class="comment">// The entry must be known not to be expunged.</span></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="params">(e *entry)</span> <span class="title">storeLocked</span><span class="params">(i *<span class="keyword">interface</span>&#123;&#125;)</span></span> &#123;</span><br><span class="line">	atomic.StorePointer(&amp;e.p, unsafe.Pointer(i))</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// LoadOrStore returns the existing value for the key if present.</span></span><br><span class="line"><span class="comment">// Otherwise, it stores and returns the given value.</span></span><br><span class="line"><span class="comment">// The loaded result is true if the value was loaded, false if stored.</span></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="params">(m *Map)</span> <span class="title">LoadOrStore</span><span class="params">(key, value <span class="keyword">interface</span>&#123;&#125;)</span> <span class="params">(actual <span class="keyword">interface</span>&#123;&#125;, loaded <span class="keyword">bool</span>)</span></span> &#123;</span><br><span class="line">	<span class="comment">// Avoid locking if it&#x27;s a clean hit.</span></span><br><span class="line">	read, _ := m.read.Load().(readOnly)</span><br><span class="line">	<span class="keyword">if</span> e, ok := read.m[key]; ok &#123;</span><br><span class="line">		actual, loaded, ok := e.tryLoadOrStore(value)</span><br><span class="line">		<span class="keyword">if</span> ok &#123;</span><br><span class="line">			<span class="keyword">return</span> actual, loaded</span><br><span class="line">		&#125;</span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">	m.mu.Lock()</span><br><span class="line">	read, _ = m.read.Load().(readOnly)</span><br><span class="line">	<span class="keyword">if</span> e, ok := read.m[key]; ok &#123;</span><br><span class="line">		<span class="keyword">if</span> e.unexpungeLocked() &#123;</span><br><span class="line">			m.dirty[key] = e</span><br><span class="line">		&#125;</span><br><span class="line">		actual, loaded, _ = e.tryLoadOrStore(value)</span><br><span class="line">	&#125; <span class="keyword">else</span> <span class="keyword">if</span> e, ok := m.dirty[key]; ok &#123;</span><br><span class="line">		actual, loaded, _ = e.tryLoadOrStore(value)</span><br><span class="line">		m.missLocked()</span><br><span class="line">	&#125; <span class="keyword">else</span> &#123;</span><br><span class="line">		<span class="keyword">if</span> !read.amended &#123;</span><br><span class="line">			<span class="comment">// We&#x27;re adding the first new key to the dirty map.</span></span><br><span class="line">			<span class="comment">// Make sure it is allocated and mark the read-only map as incomplete.</span></span><br><span class="line">			m.dirtyLocked()</span><br><span class="line">			m.read.Store(readOnly&#123;m: read.m, amended: <span class="literal">true</span>&#125;)</span><br><span class="line">		&#125;</span><br><span class="line">		m.dirty[key] = newEntry(value)</span><br><span class="line">		actual, loaded = value, <span class="literal">false</span></span><br><span class="line">	&#125;</span><br><span class="line">	m.mu.Unlock()</span><br><span class="line"></span><br><span class="line">	<span class="keyword">return</span> actual, loaded</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// tryLoadOrStore atomically loads or stores a value if the entry is not</span></span><br><span class="line"><span class="comment">// expunged.</span></span><br><span class="line"><span class="comment">//</span></span><br><span class="line"><span class="comment">// If the entry is expunged, tryLoadOrStore leaves the entry unchanged and</span></span><br><span class="line"><span class="comment">// returns with ok==false.</span></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="params">(e *entry)</span> <span class="title">tryLoadOrStore</span><span class="params">(i <span class="keyword">interface</span>&#123;&#125;)</span> <span class="params">(actual <span class="keyword">interface</span>&#123;&#125;, loaded, ok <span class="keyword">bool</span>)</span></span> &#123;</span><br><span class="line">	p := atomic.LoadPointer(&amp;e.p)</span><br><span class="line">	<span class="keyword">if</span> p == expunged &#123;</span><br><span class="line">		<span class="keyword">return</span> <span class="literal">nil</span>, <span class="literal">false</span>, <span class="literal">false</span></span><br><span class="line">	&#125;</span><br><span class="line">	<span class="keyword">if</span> p != <span class="literal">nil</span> &#123;</span><br><span class="line">		<span class="keyword">return</span> *(*<span class="keyword">interface</span>&#123;&#125;)(p), <span class="literal">true</span>, <span class="literal">true</span></span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">	<span class="comment">// Copy the interface after the first load to make this method more amenable</span></span><br><span class="line">	<span class="comment">// to escape analysis: if we hit the &quot;load&quot; path or the entry is expunged, we</span></span><br><span class="line">	<span class="comment">// shouldn&#x27;t bother heap-allocating.</span></span><br><span class="line">	ic := i</span><br><span class="line">	<span class="keyword">for</span> &#123;</span><br><span class="line">		<span class="keyword">if</span> atomic.CompareAndSwapPointer(&amp;e.p, <span class="literal">nil</span>, unsafe.Pointer(&amp;ic)) &#123;</span><br><span class="line">			<span class="keyword">return</span> i, <span class="literal">false</span>, <span class="literal">true</span></span><br><span class="line">		&#125;</span><br><span class="line">		p = atomic.LoadPointer(&amp;e.p)</span><br><span class="line">		<span class="keyword">if</span> p == expunged &#123;</span><br><span class="line">			<span class="keyword">return</span> <span class="literal">nil</span>, <span class="literal">false</span>, <span class="literal">false</span></span><br><span class="line">		&#125;</span><br><span class="line">		<span class="keyword">if</span> p != <span class="literal">nil</span> &#123;</span><br><span class="line">			<span class="keyword">return</span> *(*<span class="keyword">interface</span>&#123;&#125;)(p), <span class="literal">true</span>, <span class="literal">true</span></span><br><span class="line">		&#125;</span><br><span class="line">	&#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// Delete deletes the value for a key.</span></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="params">(m *Map)</span> <span class="title">Delete</span><span class="params">(key <span class="keyword">interface</span>&#123;&#125;)</span></span> &#123;</span><br><span class="line">	read, _ := m.read.Load().(readOnly)</span><br><span class="line">	e, ok := read.m[key]</span><br><span class="line">	<span class="keyword">if</span> !ok &amp;&amp; read.amended &#123;</span><br><span class="line">		m.mu.Lock()</span><br><span class="line">		read, _ = m.read.Load().(readOnly)</span><br><span class="line">		e, ok = read.m[key]</span><br><span class="line">		<span class="keyword">if</span> !ok &amp;&amp; read.amended &#123;</span><br><span class="line">			<span class="built_in">delete</span>(m.dirty, key)</span><br><span class="line">		&#125;</span><br><span class="line">		m.mu.Unlock()</span><br><span class="line">	&#125;</span><br><span class="line">	<span class="keyword">if</span> ok &#123;</span><br><span class="line">		e.<span class="built_in">delete</span>()</span><br><span class="line">	&#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="params">(e *entry)</span> <span class="title">delete</span><span class="params">()</span> <span class="params">(hadValue <span class="keyword">bool</span>)</span></span> &#123;</span><br><span class="line">	<span class="keyword">for</span> &#123;</span><br><span class="line">		p := atomic.LoadPointer(&amp;e.p)</span><br><span class="line">		<span class="keyword">if</span> p == <span class="literal">nil</span> || p == expunged &#123;</span><br><span class="line">			<span class="keyword">return</span> <span class="literal">false</span></span><br><span class="line">		&#125;</span><br><span class="line">		<span class="keyword">if</span> atomic.CompareAndSwapPointer(&amp;e.p, p, <span class="literal">nil</span>) &#123;</span><br><span class="line">			<span class="keyword">return</span> <span class="literal">true</span></span><br><span class="line">		&#125;</span><br><span class="line">	&#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// Range calls f sequentially for each key and value present in the map.</span></span><br><span class="line"><span class="comment">// If f returns false, range stops the iteration.</span></span><br><span class="line"><span class="comment">//</span></span><br><span class="line"><span class="comment">// Range does not necessarily correspond to any consistent snapshot of the Map&#x27;s</span></span><br><span class="line"><span class="comment">// contents: no key will be visited more than once, but if the value for any key</span></span><br><span class="line"><span class="comment">// is stored or deleted concurrently, Range may reflect any mapping for that key</span></span><br><span class="line"><span class="comment">// from any point during the Range call.</span></span><br><span class="line"><span class="comment">//</span></span><br><span class="line"><span class="comment">// Range may be O(N) with the number of elements in the map even if f returns</span></span><br><span class="line"><span class="comment">// false after a constant number of calls.</span></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="params">(m *Map)</span> <span class="title">Range</span><span class="params">(f <span class="keyword">func</span>(key, value <span class="keyword">interface</span>&#123;&#125;)</span> <span class="title">bool</span>)</span> &#123;</span><br><span class="line">	<span class="comment">// We need to be able to iterate over all of the keys that were already</span></span><br><span class="line">	<span class="comment">// present at the start of the call to Range.</span></span><br><span class="line">	<span class="comment">// If read.amended is false, then read.m satisfies that property without</span></span><br><span class="line">	<span class="comment">// requiring us to hold m.mu for a long time.</span></span><br><span class="line">	read, _ := m.read.Load().(readOnly)</span><br><span class="line">	<span class="keyword">if</span> read.amended &#123;</span><br><span class="line">		<span class="comment">// m.dirty contains keys not in read.m. Fortunately, Range is already O(N)</span></span><br><span class="line">		<span class="comment">// (assuming the caller does not break out early), so a call to Range</span></span><br><span class="line">		<span class="comment">// amortizes an entire copy of the map: we can promote the dirty copy</span></span><br><span class="line">		<span class="comment">// immediately!</span></span><br><span class="line">		m.mu.Lock()</span><br><span class="line">		read, _ = m.read.Load().(readOnly)</span><br><span class="line">		<span class="keyword">if</span> read.amended &#123;</span><br><span class="line">			read = readOnly&#123;m: m.dirty&#125;</span><br><span class="line">			m.read.Store(read)</span><br><span class="line">			m.dirty = <span class="literal">nil</span></span><br><span class="line">			m.misses = <span class="number">0</span></span><br><span class="line">		&#125;</span><br><span class="line">		m.mu.Unlock()</span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">	<span class="keyword">for</span> k, e := <span class="keyword">range</span> read.m &#123;</span><br><span class="line">		v, ok := e.load()</span><br><span class="line">		<span class="keyword">if</span> !ok &#123;</span><br><span class="line">			<span class="keyword">continue</span></span><br><span class="line">		&#125;</span><br><span class="line">		<span class="keyword">if</span> !f(k, v) &#123;</span><br><span class="line">			<span class="keyword">break</span></span><br><span class="line">		&#125;</span><br><span class="line">	&#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="params">(m *Map)</span> <span class="title">missLocked</span><span class="params">()</span></span> &#123;</span><br><span class="line">	m.misses++</span><br><span class="line">	<span class="keyword">if</span> m.misses &lt; <span class="built_in">len</span>(m.dirty) &#123;</span><br><span class="line">		<span class="keyword">return</span></span><br><span class="line">	&#125;</span><br><span class="line">	m.read.Store(readOnly&#123;m: m.dirty&#125;)</span><br><span class="line">	m.dirty = <span class="literal">nil</span></span><br><span class="line">	m.misses = <span class="number">0</span></span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="params">(m *Map)</span> <span class="title">dirtyLocked</span><span class="params">()</span></span> &#123;</span><br><span class="line">	<span class="keyword">if</span> m.dirty != <span class="literal">nil</span> &#123;</span><br><span class="line">		<span class="keyword">return</span></span><br><span class="line">	&#125;</span><br><span class="line"></span><br><span class="line">	read, _ := m.read.Load().(readOnly)</span><br><span class="line">	m.dirty = <span class="built_in">make</span>(<span class="keyword">map</span>[<span class="keyword">interface</span>&#123;&#125;]*entry, <span class="built_in">len</span>(read.m))</span><br><span class="line">	<span class="keyword">for</span> k, e := <span class="keyword">range</span> read.m &#123;</span><br><span class="line">		<span class="keyword">if</span> !e.tryExpungeLocked() &#123;</span><br><span class="line">			m.dirty[k] = e</span><br><span class="line">		&#125;</span><br><span class="line">	&#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="params">(e *entry)</span> <span class="title">tryExpungeLocked</span><span class="params">()</span> <span class="params">(isExpunged <span class="keyword">bool</span>)</span></span> &#123;</span><br><span class="line">	p := atomic.LoadPointer(&amp;e.p)</span><br><span class="line">	<span class="keyword">for</span> p == <span class="literal">nil</span> &#123;</span><br><span class="line">		<span class="keyword">if</span> atomic.CompareAndSwapPointer(&amp;e.p, <span class="literal">nil</span>, expunged) &#123;</span><br><span class="line">			<span class="keyword">return</span> <span class="literal">true</span></span><br><span class="line">		&#125;</span><br><span class="line">		p = atomic.LoadPointer(&amp;e.p)</span><br><span class="line">	&#125;</span><br><span class="line">	<span class="keyword">return</span> p == expunged</span><br><span class="line">&#125;</span><br><span class="line"></span><br></pre></td></tr></table></figure>

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